Move large blobs to external

Once externals are implemented in datajoint, we will need to move large blobs from here (particularly reso/meso.Fluorescence.Trace, reso/meso.Activity.Trace and the stack.CorrectedStack to externals). Will alleviate some weight in the db. Some of https://github.com/cajal/stimuli tables (like stimulus.Frame) could also be transferred to external